[gitlab] Update base image to Ubuntu bionic + some improvements.

We move gitlab runners to Ubuntu 18.04 "Bionic"; this allows us to
install most base dependencies using APT, and opens up the door to
saving quite a bit of time by creating a custom docker image
[c.f. #7383]

This change comes with an update of dependencies; we tweak them. Also:

- we add a more precise cache `key` constraint; this is still done
  manually, we should develop an automated way in another PR. The
  format is "$image-v$date-$hour-$minute"

- we export DEBIAN_FRONTEND=noninteractive as to avoid problems with
  package installs that ask for interactive input.

- we install Sphinx Python packages using `apt` save for
  python3-antlr4, which is still unpackaged [see
  https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=897129]

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